Cover glass for watches, mobile communication devices, displays and computing devices is commonly decorated with one or more layers of ink, paint or other materials. Decoration is done for graphic effect and/or to reduce or modify the transparency of the cover glass. Although commonly referred to as “cover glass”, the covering may be any one of a number of transparent media. Also, the decoration may be composed of layers of various materials that have been applied by a variety of means including screen print, direct print, stenciling, among others.
In some applications it is desirable to continue decoration onto the edges of the transparent media. This may include decorating edge features such as chamfers or other edge finishing details. In a manufacturing process, it is generally desirable to inspect decoration features for various attributes, for example, opacity, edge quality, and location. This inspection typically includes portions of decoration that extend onto the edges of the transparent media.
Currently, inspection of the edges of transparent media is commonly practiced for the detection and measurement of chips, cracks and other defects. Existing edge inspection systems are designed to emphasize and enhance the appearance of edge contours and common defects such as chips, scratches, and cracks. This type of inspection is typically accomplished prior to decoration. However, since parts with minor imperfections (chips, scratches, cracks and the like) may still be considered acceptable, inspection systems intended to inspect decoration should be able to work well with moderately damaged parts. For various reasons described in more detail below, inspection systems that are intended primarily for inspecting edge quality generally do not work well when used to inspect decoration of the edges. For example, these types of inspection systems typically make defects highly visible, which may obscure and make the decoration less visible for inspection. Additionally, this type of inspection system may make it difficult to discriminate defects in decoration from other types of defects.
In inspection, different conventional imaging methods may be employed, such as: backlighting, which tends to make defects appear as dark features; dark field lighting (oblique light), which tends to make defects appear as bright features; front lighting; and structured lighting, which reveals the edge profile.
Conventional backlighting can be problematic because edge contours and surface defects may direct light away from the image sensor resulting in dark areas in the image. As such, a dark line will generally appear along the edge of the glass, which interferes with the measurement of the coverage of decoration at the edge of the glass. As well, since illumination passes through the edge contours on both sides, the edge contour opposite the one being inspected may have an undue effect on the resulting image.
Conventional dark field lighting can be problematic because the margins of the decoration along an edge contour may be essentially invisible due to the decorations being far from normal to the direction of image acquisition. Also, dark field lighting emphasizes minor defects such as surface texture, scratches, cracks, and dust, which detracts from locating decoration edge contours. Additionally, dark field lighting cannot generally provide a measure of opacity.
Conventional front lighting is generally not suitable for decoration inspection because it is affected by gloss and highlights, particularly if the decoration is wet. In-line inspection is similarly not suitable because the decoration is typically freshly applied and still wet and glossy, while various artifacts, such as gas bubbles and screen print fabric texture, have not yet leveled out.
Conventional side lighting, wherein the transparent media is illuminated from the side, is also generally not suitable because this type of lighting may result in uneven contrast due to variable gloss and can result in flares from edge chips. These detrimental outcomes are particularly noticeable on dark or black decoration. Additionally, front lighting and side lighting are generally not desirable as they are unable to provide a measure of opacity, which is typically used to determine the thickness of decoration.
In conventional systems, decoration is typically inspected with backlight since, in addition to revealing location and quality of patterned features, backlighting permits the opacity of the decoration to be judged. Additionally, multi-spectral illumination and/or optical sensing may be used to measure spectral transmittance. Front lighting may also be used in order to also measure decoration color and/or reflectance. Typically, this sort of inspection is not done at or near edges and particularly places where the decoration comes near or wraps around the edges as these inspection systems generally do not perform well in these locations.
Some edge inspection systems use structured light in order to inspect the edges of a glass. This type of system resolves the edge into a 3-D profile. However, this type of system may not be able to determine the quality of decoration in the edge area because the edge area is typically contoured (by, for example, a chamfer or radius) and decoration layers are typically only a few microns thick. Additionally, the edge area may have some surface imperfections due to machining which may produce edge defects, such as small chips, that are within acceptable ranges for cover glass quality control. As well, this type of inspection system cannot make a measurement of opacity. Furthermore, while the important aspect of decoration coverage is the deposition of the more opaque pigments, binders tend to migrate from the decoration which makes edge contours less distinct.
Typically, due to the aforementioned reasons, existing inspection systems used to assess transparent media quality, and in particular the quality of decoration, may not be sufficient to accurately qualify decoration over edge features and accurately measure decoration coverage. As such an improved inspection system and method are required.